This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2000-263592, filed Aug. 31, 2000, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a waveform measuring apparatus, in particular, to a waveform measuring apparatus for determining a signal waveform of a signal under test having an arbitrary repetition cycle which is input.
2. Description of the Related Art
Generally, a signal generator for generating a signal under test such as an electric signal, an optical signal or the like having an arbitrary repetition cycle incorporates a reference signal oscillator for generating a reference signal having a reference frequency fs, and a waveform pattern generation portion for generating a waveform pattern of the signal under test.
Then, in such signal generator, a repetition frequency signal having a designated repetition frequency fa is created by using a reference signal output from the reference signal oscillator while an electric signal and an optical signal are created which have an arbitrary repetition cycle Ta by using this repetition frequency signal and the waveform pattern output from the waveform pattern generation portion.
The electric signal and the optical signal having a repetition cycle Ta output from such signal generator are generally incorporated in an information communication system and are used as a signal under test of various communication devices including, for example, optical transmission cable.
Therefore, it is necessary to measure in detail the characteristic of the electric signal and the optical signal output from the signal generator prior to the practice of the test of various communication devices including the light transmission cable incorporated in the information communication system.
As one characteristic of this electric signal and the optical signal, the signal waveform is measured.
Conventionally, there are proposed various measuring methods for measuring a signal waveform of the signal under test that is an electric signal, an optical signal or the like having such an arbitrary repetition cycle.
However, in the case of a high frequency signal having a repetition cycle Ta, namely, a repetition frequency fa exceeding 10 GHz, the method for measuring the waveform of the signal under test is used a sampling method.
A representative sampling method for measuring a signal waveform of the signal under test which has this repetition frequency fa exceeding 10 GHz will be explained by using FIGS. 6A, 6B and 6C.
As shown in FIGS. 6A and 6B, the signal under test xe2x80x9caxe2x80x9d which has this repetition cycle Ta (for example, a repetition frequency fa=10 GHz) is sampled with a sampling signal b having a frequency Tb (for example, repetition frequency fb=999,9 MHz) longer than a repetition cycle Ta of this signal under test xe2x80x9caxe2x80x9d.
In this case, it is so constituted that as shown in FIGS. 6A and 6B, the sampling position of the sampling signal b to the signal waveform having the repetition cycle Ta of this signal under test xe2x80x9caxe2x80x9d is shifted by a small time xcex94T with the passage of time by adjusting a relationship between repetition cycles Ta and Tb with the result that the sampling position is delayed as seen in xcex94T, 2xcex94T, 3xcex94T, 4xcex94T, 5xcex94T, 6xcex94T . . . .
Consequently, the signal under test c after being sampled with this sampling signal b comes to have a discrete waveform in which a pulse-like waveform appears at a position synchronous with the sampling signal b as shown in FIG. 6C.
Then, the envelope waveform of each pulse-like waveform becomes a signal waveform d which is expanded in a direction of time axis of the signal under test xe2x80x9caxe2x80x9d.
A waveform measuring apparatus for measuring the signal waveform d of the signal under test xe2x80x9caxe2x80x9d in the principle of sampling technique shown in FIGS. 6A, 6B, 6C is constituted, for example, as shown in FIG. 7.
The signal under test xe2x80x9caxe2x80x9d which has a repetition frequency fa (repetition cycle Ta) is input to a sampling cycle 1 and a frequency divider 2.
The frequency divider 2 sends an output signal obtained by dividing the repetition frequency fa of the signal under test xe2x80x9caxe2x80x9d to 1/n to the phase comparator 3.
The voltage control oscillator (VCO) 4 functions as a phase locked loop (PLL) which generates a signal having a frequency (fa/n) having a frequency of 1/n (n: positive integer) of the repetition frequency to feed back the signal to the phase comparator 3.
The phase comparator 3 which constitutes a phase locked loop (PLL) together with the voltage control oscillator (VCO) 4 detects a phase difference between the phase of the output signal of the voltage control oscillator (VCO) 4 and a phase of the output signal of the frequency divider 2 and sends the phase difference to the voltage control oscillator (VCO) 4 as a phase difference signal.
With this phase locked loop (PLL), the phase of the output signal from the voltage control oscillator (VCO) 4 is synchronized with the phase of the signal under test xe2x80x9caxe2x80x9d.
The frequency (fa/n) of the output signal having a frequency (fa/n) output from the voltage control oscillator (VCO) 4 is converted into a frequency of (fa/n) xe2x88x92xcex94f by a fixed dividing rate of frequency divider 5a and a fixed multiplying rate of frequency multiplier 5b to be input to the sampling signal generation circuit 6.
Here, the sampling signal generation circuit 6 applies a sampling signal b having a repetition frequency (fb) as shown in an equation (1) which is synchronized with the output signal which is input and a repetition cycle (Tb) as shown in the equation (2) to the sampling circuit 1.
fb=(fa/n)xe2x88x92xcex94fxe2x80x83xe2x80x83(1) 
Tb=(nTa)+xcex94Txe2x80x83xe2x80x83(2) 
However, the relationship between xcex94f and xcex94T can be approximately shown in the equation (3).
xcex94f/xcex94T=fa2/n2xe2x80x83xe2x80x83(3) 
Then, the sampling circuit 1 sends a signal under test c which is sampled by sampling the signal under test xe2x80x9caxe2x80x9d which has been input in synchronization with the sampling signal b input from the sampling signal generation circuit 6 to the next signal processing/waveform display portion 7.
This signal processing/waveform display portion 7 calculates an envelope waveform of the signal under test c after being sampled while converting a magnification of the time axis of this envelope waveform into the magnification of the original signal under test xe2x80x9caxe2x80x9d to be displayed and output as a signal waveform d of the original signal under test xe2x80x9caxe2x80x9d.
In this case, the expansion ratio of the envelope waveform measured with respect to the signal waveform d of the signal under test xe2x80x9caxe2x80x9d is (fa/nxcex94f).
Incidentally, in the case where the signal under test xe2x80x9caxe2x80x9d is not an electric signal but is an optical signal, this optical signal is converted into an electric signal to be applied to the frequency divider 2.
Furthermore, in the case where the signal under test xe2x80x9caxe2x80x9d is not an electric signal but is an optical signal, for example, an electro-absorption modulator is used instead of the sampling circuit 1.
This electro-absorption modulator is capable of sampling a pulse-like signal under test xe2x80x9caxe2x80x9d that is an input optical signal by applying a pulse-like electric field that is a sampling signal to the electro-absorption modulator.
Then, the signal under test c that is an optical signal which is sampled is sent to the signal processing/waveform display portion 7 after being converted into an electric signal.
However, the following problems to be settled are provided even in a conventional waveform measuring apparatus using a sampling technique shown in FIG. 7.
That is, an output signal from the fixed multiplying rate of frequency multiplier 5b for creating a sampling signal b having a repetition signal fb (fa/n)xe2x88x92xcex94f output from the sampling signal generation circuit 6 is created with a phase locked loop (PLL) circuit comprising a fixed dividing rate of frequency divider 2 for dividing the signal under test xe2x80x9caxe2x80x9d, the phase comparator 3 and the voltage control oscillator (VCO) 4.
In this manner, the sampling signal b is an equivalent to that is created by processing the signal under test xe2x80x9caxe2x80x9d which is an object of measurement with the result that such sampling signal b is constantly phase synchronized with the signal under test xe2x80x9caxe2x80x9d.
Consequently, the jitter generation amount in the timing of sampling to the signal waveform d of the signal under test xe2x80x9caxe2x80x9d is suppressed, so that the measurement precision of the signal waveform d of the signal under test xe2x80x9caxe2x80x9d is improved.
However, the repetition frequency fb of the sampling signal b is represented in a function of a repletion frequency fa of the signal under test xe2x80x9caxe2x80x9d as apparent from the above equations (1) and (2).
This fact means that the repetition frequency fb of the sampling signal b cannot be arbitrarily set independently of the repetition frequency fa of the signal under test xe2x80x9caxe2x80x9d when using the fixed dividing rate of frequency divider and the fixed multiplying rate of frequency multiplier.
That is, in the conventional waveform measuring apparatus, as shown in FIG. 7, when the repetition frequency fa of the signal under test xe2x80x9caxe2x80x9d changes, the time resolution of the signal waveform d of the signal under test xe2x80x9caxe2x80x9d, namely the measurement precision automatically changes.
So that the signal waveform d of the signal under test xe2x80x9caxe2x80x9d cannot be measured in an arbitrary time resolution.
Furthermore, since the sampling signal b is directly created from the signal under test xe2x80x9caxe2x80x9d, there is a problem in that a complicated circuit structure is required which comprises the frequency divider 2, the phase comparator 3, the voltage control oscillator (VCO) 4, the frequency divider 5a and the multiplier 5b.
In view of the above situation, an object of the present invention is to provide a waveform measuring apparatus which is capable of improving a measurement precision of a signal waveform of a signal under test and is capable of measuring the signal waveform in an arbitrary resolution precision because a frequency of a sampling signal for sampling the signal under test can be arbitrarily set independently of a repetition frequency of the signal under test by measuring the repetition and creating the sampling signal by using a common reference signal.
The present invention can be applied to the waveform measuring apparatus for sampling the signal under test which has an arbitrary repetition cycle which is input with a sampling signal having a cycle longer than the repetition cycle of the signal under test to determine an envelope waveform of the signal under test which is sampled, the apparatus determining the signal waveform of the signal under test from this envelope waveform.
In order to attain the above object, there is provided a waveform measuring apparatus (1) having sampling signal generation means (16) for generating a sampling signal having a cycle longer than a repetition cycle of a signal under test, a sampling portion (12) for sampling the signal under test in synchronization with the sampling signal from the sampling signal generation means and data processing portion (23) for determining an envelope waveform of a signal under test which is sampled with the sampling portion, and determining a signal waveform of the signal under test from this envelope waveform; the apparatus comprising:
reference signal generation means (14) for generating a reference signal independently of a repetition cycle of the signal under test;
frequency measuring means (15) for measuring a repetition frequency of the signal under test by using a reference signal from the reference signal generation means; and
sampling frequency setting means (20) for computing and setting a value of a frequency of the sampling signal which can obtain a desired delay time with respect to a phase of the signal under test based on a value of a repetition frequency measured with the frequency measuring means;
wherein the sampling signal generation means uses the reference signal from the reference signal generation means and the value of the frequency set by the sampling frequency setting means to generate a sampling signal having a cycle corresponding to the frequency.
In the waveform measuring apparatus which is constituted in this manner, the reference signal generation means (14) generates a reference signal independently of the repetition cycle of the signal under test.
The frequency measuring means (15) measures the repetition frequency of the signal under test by using a reference signal from the reference signal generation means.
The sampling signal frequency setting means (20) sets a frequency of a sampling signal which can obtain a desired delay time with respect to a phase of the signal under test by using a repetition frequency measured with the frequency measuring means.
Consequently, the repetition frequency (repetition cycle) of the signal under test is accurately measured with the frequency measuring means.
Then, the sampling frequency setting means sets a frequency of the sampling signal which can obtain a desired delay time with respect to the phase of the signal under test by using the repetition frequency measured with the frequency measuring means.
Then, the sampling signal generation means creates a sampling signal having a cycle of the frequency which is set so that a desired delay time can be obtained with respect to the phase of the signal under test.
In this case, since it is possible to set the frequency of the sampling signal in an arbitrary relation with respect to the repetition frequency of the signal under test so that the signal waveform of the signal under test can be measured in an arbitrary resolution.
Furthermore, the repetition frequency of the signal under test is measured and the sampling signal is created by using a common signal.
Consequently, with respect to the sampling signal, the set state of the frequency set in advance with respect to the signal under test can be accurately measured so that the precision in the measurement of the waveform can be improved.
Furthermore, in order to attain the above object, according to the present invention, there is provided a waveform measuring apparatus (2) according to (1), wherein the reference signal generation means includes a rubidium atomic oscillator.
Furthermore, in order to attain the above object, according to the present invention, there is provided a waveform measuring apparatus (3) according to (1), wherein the reference signal generation means includes a cesium oscillator.
In order to attain the above object, according to the present invention, there is provided a waveform measuring apparatus (4) according to (1), further comprising:
a power divider for dividing the signal under test which is an optical signal into two directions when the signal under test is the optical signal; and
a photo detector for converting a signal under test which is one optical signal which is divided with the power divider into a signal under test of an electric signal;
wherein the repetition frequency of the signal under test which is converted into an electric signal by the photo detector is measured with the frequency measuring means,
the measured value of the repetition frequency of the signal under test measured with the frequency means is given to the sampling frequency setting means.
Furthermore, in order to attain the above object, there is provided a waveform measuring apparatus (5) according to (1), further comprising:
a power divider for dividing the signal under test which is an optical signal into two directions when the signal under test is the optical signal; and
a clock recovery for converting a signal under test which is an optical signal into a signal under test of an electric signal having a repetition frequency and outputting the signal by detecting a clock of the repetition cycle from one signal under test divided with the power divider;
wherein the repetition frequency of the signal under test which is converted into an electric signal with the clock recovery is measured with the frequency measuring means, and
the measured value of the repetition frequency of the signal under test measured with the frequency measuring means is given to the sampling frequency setting means.
Furthermore, in order to attain the above object, according to the present invention, there is provided a waveform measuring apparatus (6) according to (1) further comprising:
a photo detector (21) for receiving a signal under test of an optical signal sampled with a sampling signal input from the sampling signal generation circuit with the electro-absorption modulator and converting the signal under test which is an optical signal after being sampled into a signal under test which is an electric signal when the signal under test is an optical signal and the sampling portion is an electro-absorption modulator;
an analog/digital converter (22) for converting the signal under test which is converted into an electric signal with the photo detector into a signal under test to send the converted signal to the data processing portion; and
a display device (24) converting a magnification of a time axis in the envelope waveform determined with the data processing portion into a magnification of the original signal under test to display the magnification as a signal waveform of the signal under test.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.